[pxe] If no ProxyDHCPACK exists, use DHCPACK for the fake ProxyDHCPACK packet

WinPE seems to have a bug that causes it to always use the TFTP server
IP address and filename from the ProxyDHCPACK packet, even if the
ProxyDHCPACK packet doesn't exist.  This causes it to end up
attempting to fetch a file such as

  tftp://0.0.0.0/bootmgr.exe

If we don't have a ProxyDHCPACK to use, we pretend that it was a copy
of the DHCPACK packet.  This works around the problem, and hopefully
won't surprise any NBPs.

src/net/fakedhcp.c

@@ -181,11 +181,9 @@ int create_fakeproxydhcpack ( struct net_device *netdev,
 	/* Identify ProxyDHCP settings */
 	settings = find_settings ( PROXYDHCP_SETTINGS_NAME );
 
-	/* No ProxyDHCP settings => return empty block */
-	if ( ! settings ) {
-		memset ( data, 0, max_len );
-		return 0;
-	}
+	/* No ProxyDHCP settings => use normal DHCPACK */
+	if ( ! settings )
+		return create_fakedhcpack ( netdev, data, max_len );
 
 	/* Create base DHCPACK packet */
 	if ( ( rc = dhcp_create_packet ( &dhcppkt, netdev, DHCPACK, NULL,